1. Field of the Invention
The present invention is directed to a solid state image converter of the type having light-sensitive cells arranged in a matrix, the cells comprising oppositely-connected diodes, with at least one diode being a photodiode, and having driver circuits for driving the diodes, the diodes being connected between the respective row and column lines of the driver circuits. Such solid state image converters serve the purpose, for example, of acquiring x-ray images in radiology.
2. Description of the Prior Art
For example, U.S. Pat. No. 4,948,966 discloses such a solid state image converter wherein two diodes are connected with opposite polarity between the row and column lines of the drive circuits. The photodiodes are operated in a storage mode using their self-capacitance, so that they can be sequentially read out after an exposure. It must be assured for real-time radiological image display that the read-out can ensue quickly enough and that image information of successive images (frames) do not mix.
For this reason, a reset that ensues by illuminating the matrix is provided between two read-out events. As a result, the photodiodes are completely discharged and rendered low-impedance. Since the illumination cannot ensue row-by-row, a simultaneous, common resetting of the entire array is undertaken. Due to the subsequent row-by-row, sequential read-out, however, time intervals of different lengths arise between the common resetting and the read-out of the respective rows.
In an x-ray diagnostics installation, the x-ray image converter can be composed of such a photodiode matrix on which a scintillator layer is applied, for converting the incident x-radiation into light. In order to avoid measuring times of different lengths, irradiation can be carried out with a switchable x-ray tube that enables an equally long illumination of all diodes of the array. The switchable x-ray tube with its associated voltage generator and the light source for the resetting, however, represent a considerable outlay.
German PS 35 46 487 discloses a solid state image sensor wherein the picture elements, arranged in a matrix, are each composed of a field effect transistor and the matrix is driven by row and column scanning circuits. During the illumination period, the source and drain regions are reverse biased. During the read-out period, the source as well as drain regions are connected to ground potential.
German OS 39 51 547 discloses an apparatus for driving an image pick-up element that is composed of a CCD image converter. The matrix-like photodiodes emit their output signal charges to a transfer CCD after the expiration of the illumination time, so that the charges can be subsequently read out.